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Miscibility Gaps

A miscibility gap can occur when the Gibbs energy of the phase goes through a maximum over some part of its composition range. Unless the user specifies the possibility of a miscibility gap, MTDATA may find an apparent equilibrium involving the minimum on the “wrong” side of the maximum. Miscibility gaps can occur, for example for regular solutions if the regular interaction parameter is greater than 4RT and also in sublattice reciprocal systems in which one pair of compounds is dominant. The solution adopted in MTDATA is for the user to include the relevant phases in the calculation twice or, if necessary, three times. Calculations become slower as a result. Miscibility gaps may also be pre-assigned in the database.

It is usually possible to tell if there is a problem because, if the potential for a miscibility gap has been ignored, MTDATA will tend to give irreproducible results. If the status of all the phases is normal with no miscibility gaps declared, the diagram produced will vary with the starting point of the calculation and parts of the diagram may appear not to fit together. The Cu-Fe system is a typical case and will serve as an example.
The liquidus in the Cu-Fe system is rather flat which indicates a metastable miscibility gap in the liquid at a sub-liquidus temperature. Use the right mouse button in the calculated diagram to find out what phases are present and consider introducing miscibility gaps for some of these. Because both Fe and Cu have liquid and fcc phases these are the obvious pair to try first and indeed setting the miscibility gap parameter to 1 for both of these phases is sufficient, whereas setting the miscibility gap status to 1 for either one but not the other would not have been sufficient. It would be wise as a precaution to introduce a miscibility gap for the bcc phase also.

The Gplot module of the command line version of MTDATA will provide definite information on the shape of the Gibbs energy surface in binary systems.

Method of changing the miscibility gap status of a phase

Select the phase tab and double click on a phase in the Normal list of the System Window. The phase properties box will open. The number of miscibility gaps in a phase is usually initially zero but can be changed to 1 or 2 or back to 0 using the combo box. In some cases phases have pre-assigned miscibility gaps, for example in the liquid phase of the lime-silica system in the NPL oxide database. When the phase properties window is closed, if the number of miscibility gaps has been chosen to be 1 or 2, a number, either 2 or 3 respectively, will appear to the left of the phase name. This number is the maximum number of versions of the phase that can coexist. Two miscibility gaps, ie 3 versions of the phase, can occur in some phases, particularly those with sublattices.

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Updated 7 May 2010